The difference between getting invitation and stealing a piece is that you get ideas about the pice but put your own spin on the art. I would think it would be okay to use another picture but change something about it and also give the original source credit because playdarizing is something you don't play with.
The two things I combined was a eye and fish. The eye is crying but instead of tears it's fish! I used water color to match the inspiration of tears to give it more of a watery affect. I would change how small it is. I wish it was a little longer so I could paint more fish to make it more realistic. The one thing I found that was really successful was the iris of the eye, I loved the idea of putting that as the only color in the whole piece. I love the effect it gives the artwork!